Three main families of protein-crosslinking agents are generally described.
The first family consists of mineral tannins, such as di-, tri- or polyvalent cation salts, for instance chromium, aluminium, iron, zinc, copper, cobalt, titanium, zirconium, silicon and caesium salts.
Mineral tannins are especially effective in alkaline medium. In their basic form, these polyvalent metal salts bond stably with the free carboxylic functions of certain amino acids of proteins.
Their field of application is in particular that of the tanning of biodegradable or renewable protein materials, in particular that of the production of leather by tanning the collagen constituting the dermis of the animal skin.
The aim of this operation is to convert the skin to leather, i.e. to convert a putrescible material into an imputrescible material, by taking away from the microorganisms the possibility of attacking the collagen fibres.
In the case of chromium tanning, which represents approximately 90% of the tanned hides in the world, the chromium compounds attach to the proteins by forming complexes:                either with the free carboxylic groups of the amino chains of said proteins,        or, by means of sulphate, formate or oxalate groups, with the free amine functions of the amino chains of said proteins.        
Crosslinking, i.e. bonding of two or more peptide chains, by means of the chromium compounds, then occurs.
These mineral tannins are particularly effective but generate large amounts of waste that are prejudicial to the environment.
Mineral tannins are sometimes also used in the field of tanning proteins intended for animal feed, in the form of zinc salts or salts of other cations. Excessive consumption of such tanned proteins is to be avoided since it can result in toxic effects for the animal.
The second family of tannins consists of synthetic tannins such as epichlorohydrin resins, polyamide-amine-epichlorohydrin resins, polyethyleneimine resins and isocyanate resins such as methylenediphenyl diisocyanate.
Their field of application is in particular that of glues or adhesives, in particular wood glues or glues for corrugated cardboard.
The third family of tannins consists of aldehyde tannins, more particularly formaldehyde, glyoxal (ethanedial) or glutaraldehyde, and all the commercial derivatives thereof sold in the form of various condensates.
Aldehyde tannins bond to proteins via covalent bonds, mainly generating condensation products. A double condensation reaction may also result in the formation of crosslinked products.
These condensation products are generally derived from the reaction forming imines by condensation of the carbonyl function of the crosslinking agent with the free amine function of certain amino acids of said protein (in particular the ε-amino function of lysine).
Aldehyde tannins are in particular used in three main fields of application.
The first field of application is that of the preparation of protein materials, for example leathers, moulded items, textile fibres, pharmaceutical or food capsules, photographic films, motor car parts and buttons, from milk caseins, zeins, collagens, soybean proteins, blood from an abattoir, and gelatins, most commonly crosslinked with formaldehyde.
For leathers, the tanning with aldehyde agents consolidates the structure of the collagen by creating bonds between groups of the aldehyde tannin and at least two groups belonging to the peptide chain. Specific leathers can thus be obtained.
The second field of application is that of the tanning of proteins intended for the production of film-forming compositions for coverings, coatings or adhesives.
Use is commonly made, for example, for bonding papers, cardboards, particles, wood or other materials, of adhesives prepared from gelatin, from blood proteins, from fish proteins, from milk caseins or from plant proteins, to which formaldehyde or glyoxal is generally added in order to improve the water- and moisture-resistance of the adhesive joint and also its adhesive characteristics.
The third field of application is that of the tanning of proteins intended for feed, in particular ruminant feed, for producing “resistant” proteins.
The resistant proteins are then proteins that have been modified in such a way that they are less degraded by certain microbial enzymes in the rumen.
However, after having passed through the rumen, these proteins once again have satisfactory digestibility when they arrive in the rennet stomach and in the small intestine. The acid pH and the digestive enzymes in fact free the proteins and thus make them available again.
This tanning operation therefore allows better assimilation of the food intake by ruminants, resulting in better zootechnical performance levels (improved meat and milk productions).
However, the major drawback of the aldehyde tannins used in these three fields of application is their high toxicity.
One of the aldehydes most commonly used, because it is very reactive, is formaldehyde (also referred to as formol under its common technical name). Now, this chemical agent is among the most toxic and the most dangerous of the aldehyde tannins to handle, since it is in the gaseous state at ambient temperature.
In the field of ruminant feed, in order to remedy this use of substances that are active but too toxic, it has been proposed, for example in patent EP 284,548, to perform a tanning reaction by means of a particular category of aldehydes, e.g. aldoses.
These are xylose, ribose, mannose, lactose and glucose. These molecules are non-toxic, unlike the aldehyde tannins conventionally used.
However, the teaching of that patent reveals that it is necessary to finely control the tanning reaction so as to obtain what the authors of that patent call an “early” Maillard condensation product rather than an “intermediate” Maillard product.
It is also essential to select a food described as “orthodox”, i.e. containing a group of proteins with a free amine function present in the food in a ratio of 1.5 to 1 relative to the total proteins.
Finally, it is necessary to hydrolyse the condensation product at a pH of less than 4, and to limit the percentage of reducing sugars therein.
The process described in patent EP 284,548 is therefore much too laborious to carry out.
Besides the aldoses proposed in patent EP 284,548, a single ketose is mentioned in said patent application. This is fructose.
However, this ketose containing 6 carbon atoms is only considered because of its intermediate reducing capacity between those of glucose and mannose. That patent EP 284,548 merely cites this ketose without truly recommending it. No other ketose is used in the process described.
Moreover, to the applicant company's best knowledge, the potentialities and the chemical reactivities of other sugars carrying a ketone function have never been the subject of an examination with respect to the fields of activity with which the present invention is concerned.
The simplest ketose of the sugar series is dihydroxy-acetone (or DHA), also called 1,3-dihydroxy-2-propanone, which is a compound of formula CH2OH—CO—CH2OH.
Besides its use in the cosmetics sector, but in external applications for its properties of colouring the epidermis of the skin (“self-bronzing” effect)—used alone or combined with erythrulose—DHA is especially known for its antioxidant, odour-inhibiting and biocidal properties (antibacterial effect and antibacterial and antifungal effects).
DHA is especially recommended as a mixture with pyruvic acid, in particular for ensuring metabolic control of the lipid content of the liver and of adipocytes. It is then possible to control, by this means, the lipid/protein balance in mammals.
This use is, for example, described in U.S. Pat. No. 4,351,835, by oral administration of pyruvate and of DHA in order to avoid weight gain in mammals, or in U.S. Pat. No. 4,415,575, where the administration thereof promotes protein concentration in animals:
In order to illustrate another known property of DHA, i.e. its inhibitory effect on undesirable odours, patent application JP 50,7803 describes the use of DHA in preventing the resurgence of odours from oils or from fats.
The biocidal effect of DHA is, for its part, described, for example, in patent CA 1,054,434. It is used as a food additive, in particular for preserving foods (fish, meat, fruit, vegetables, etc.), by bringing the food into contact with DHA at ambient temperature.
As regards ketoses containing 4 or 5 carbon atoms (i.e. erythrulose, ribulose or xylulose), unlike DHA, they do not appear, according to the literature, to have been the subject of work studies on their intrinsic properties.
The latter molecules are, to the applicant company's knowledge, only used today as synthesis intermediates of products with added value, such as their corresponding hydrogenation products (erythritol, ribitol, arabitol and xylitol).
From all the above, it results that there is an unsatisfied need for having a substitute for crosslinking agents, in particular a substitute for mineral tannins, for synthetic tannins and for aldehyde tannins, that is simple to produce and to use and that is of equivalent, or even improved, effectiveness compared with the conventional tanning agents.